There are currently a number of different commercial wearing part systems comprising exchangeable wearing parts, which are detachably disposed in wearing part holders mounted on the tool of a soil-working machine, for the loosening and separation of more or less hard soil and rock materials from a working surface, whereafter these worked materials can be suitably removed. One example of such wearing part systems, tools, wearing parts and wearing part holders is here specifically constituted by the rotary cutter head of a dredger, also referred to below as a dredger cutter, with its tooth system comprising exchangeable wearing parts, also referred to as wearing teeth, which wearing teeth are detachably mounted in tooth holders. Such wearing part systems can also, of course, be used in other types of soil-working machine tool, such as in a shovel for a digger, in a rock blade or a drill bit, etc.
In the case of dredger cutters specifically, the said wearing teeth are arranged at a certain distance apart, along more or less curved arms or spiral, elongated cutter head blades which protrude in plural from a central rotation body disposed on a central hub which is rotatable via a drive shaft. The cutter head blades expediently extend helically from the hub at the front end of the rotation body and rearwards in the direction of feed of the tool to the rear end of the rotation body, normally comprising an annular part, which holds together the cutter head blades and in which there is also arranged a suction device for carrying away the loosened, worked materials via a space between the said cutter head blades.
Such tooth systems usually comprise two main coupling parts in the form of a “female part” and a “male part”, which, in mutual interaction via a common geometric form which is precisely matched for the female part and the male part, together form one piece, a composite “tooth”, i.e. the said tooth system, which composite tooth can be one in a series of teeth arranged adjacent to one another along, for example, the front edges of the cutter head blades, the cutter of a drill bit, or the sharp cutting edge of the shovel and of the rock blade. How far the female part or the male part is mounted on the tool is of minor importance, the important thing is that the two coupling parts are removable and lockable in relation to each other and that the part which constitutes the holder part is permanently fixed to the tool.
A “composite tooth” of this type therefore comprises a first coupling part, namely the abovementioned wearing part in the form of, for example, an exchangeable front wearing tooth having some form of working part, for example a tip or a cutting edge, and also comprising a fixing part, preferably its—in relation to the body or neck thereof, for example a tooth body or tooth neck—rear or lower part, for example a rear shaft or opening, for mounting in a specific groove, opening or pin, custom-made for just this type of wearing parts, in a second coupling part, i.e. the rear or lower fixed holder part, here the tooth holder. In order to achieve a dynamic, yet still reliable securement of the exchangeable wearing tooth on the tooth holder, the coupling parts also comprise a coupling system which is common to the parts and has a releasable locking mechanism. Each such coupling system has an extremely characteristic geometry, in which the respective coupling part contains its own specific solution, comprising mutually interacting surfaces and forms of the abovementioned shaft, groove, etc., one or more securing elements, for example a locking pin, and/or one or more clamping devices for realizing a clamping of the wearing part on the holder part, compare SE-524 301 (EP-1 644 588), in an attempt to get the wearing part of each “tooth” to be held fully fixed in the intended place and in the correct position in an effective, secure and functional manner, also involving just a minimal wear between the coupling parts, until the wearing part, due to the nonetheless unavoidable wear, has to be released and replaced by a new wearing part for continued use of the particular tool.
Known commercial tooth systems of this type are designed to absorb loads (F) from the use of the tool via the specially configured and mutually interacting contact zones, which are arranged along the joint between the coupling parts constituted by the shaft, the pin and the groove or opening.
It will be appreciated, however, that, during use of the tool, not only loads which are parallel with the longitudinal plane of symmetry A of the coupling geometry, but also loads which deviate from this plane of symmetry, are in action. Essentially each acting load (F) therefore comprises, see FIG. 1 and FIG. 3, firstly a shearing force component Fc, which acts substantially from the front, parallel with the working surface and substantially axially in relation to the said joint, secondly a normal force component Fs, which acts substantially from above, perpendicular to the working surface, and thirdly at least one lateral transverse force component Fp, which acts from the side or the sides, substantially parallel with the working surface and more perpendicular in relation to the extent of the said wearing tooth along the plane of symmetry A, i.e. the said tooth neck thereof, which constitutes a more strongly protruding extension of the tooth body, in front of the common joint of the coupling parts, which tooth neck, during use of the wearing part, shall project from and at a certain specific angle to the rest of the tooth body. The lateral transverse force component Fp is typically smaller than the shearing force component Fc and the normal force component Fs.
Positional terms which are used in this description, such as rear, front, lower, upper, vertical, lateral or horizontal, etc., can consequently be derived from the above-given definitions for the said forces and the mutual relationship of the coupling parts, as well as their position relative to the working surface.
The new concept for a tooth system according to the present patent application comprises a number of characteristics, which characteristics alone or in combination are unique compared with the currently known tooth systems, and which characteristics provide advantageous solutions to a number of problems which can arise in the known tooth systems.
In conventional tooth systems, it is a fact that, though the tooth systems are relatively strong, they have an over-rapid wear-down of the bearing surfaces, or other working surfaces exposed by the operation, which, for example, bear against or have a driving, transporting, penetrating, crushing, shearing, etc. effect upon the working surface. All such surfaces exposed to abrasion or wear are also referred to below as wearing surfaces, regardless of specific function. In the embodiments shown in this application, the wearing parts are of the type which are removable, yet during the work are wholly fixed in relation to the said tool, which wearing parts are fixed in the holder parts outermost on the tool, in contrast to those wearing parts which are removable but are additionally rotatable about their own longitudinal axis. It is presumed, however, that a person skilled in the art will grasp how the wearing parts according to the invention may be applied to many types of working tool, even if these are not expressly illustrated with examples herein.
In a dredger having a rotary cutter head, for example, the dredger vessel is anchored rotatably in the stern of the dredger vessel. Winches are disposed to the port and starboard of the prow of the vessel, which winches are anchored in the seabed and with which the prow of the vessel can be winched in a motion pendulating from side to side about the aft anchorage, at the same time as the cutter head is rotated about its drive shaft. In this rotary use of the wearing teeth, the tooth tip is normally worn from primarily one of its two opposite lateral sides at the front end of the tooth neck due to the said lateral transverse force component Fp, i.e. one of the two, in relation to the extent of the neck, longitudinal sides constitutes the bearing surface, or a first wearing surface, against the working surface, but since the dredger tool is also guided back and forth over the seabed in the said pendulum and sweeping motions with the aid of the winches, a wear-down of the opposite side also occurs, whereupon a second wearing surface is formed.
Since the acting force components Fp, Fs, Fc are constantly changing in strength and act from many directions, the steel can suffer fatigue, and if then the different strength properties of the steel are at the same time too low to withstand the harsh dredger work, the cast steel of the tooth tip tends to be split also into largish splinters or fragments, which very quickly wears down the whole of the tooth neck until the wearing tooth becomes ineffectual and also the tooth holder risks becoming damaged if a change is not made in time. The conventional dredger wearing parts which are currently used therefore become worn far too quickly and have to be changed and replaced with new wearing teeth far too often, resulting in expensive tooth costs and many costly operating stoppages. Similarly disadvantageous developments also prevail in other types of wearing tool. It is additionally the case that the tooth neck has a maximum possible extent, and thus a maximum working length or wear length, which is determined by, for example, maximum permitted buckling and bending load. Should the loads upon the cast steel become excessive, an over-long tooth neck will quite simply be able to be broken off and immediately render the wearing tooth totally unusable.
In order to prevent this, it is known that wearing teeth have a cross section which increases towards the base, whereby, in turn, the clearly disadvantageous characteristic is acquired that each contact surface or wearing surface becomes increasingly blunt the more the wearing surface is worn, so that the penetrative action of the wearing tooth finally becomes quite worthless.
At present, the cutter head of the dredger tool has to be raised from the water in order to be able to check which wearing teeth need changing. This means, firstly, that certain wearing teeth are changed unnecessarily, since the cutter head was up anyway and it was felt in the inspection that the wearing tooth would not last out till the next visual inspection and, secondly, that certain wearing surfaces are changed too late, so that the tooth holders in certain cases suffer serious damage. That this is very disadvantageous will be easily appreciated if one is aware that, in a typical dredger in full operation, between 4,000 and 5,000 wearing teeth are changed per week. If just 5% are changed unnecessarily, this gives a very large extra cost per week.
Another disadvantage which must here be taken into account is that the wearing tooth which is left contains valuable metal which should be recovered. If, as in certain wearing parts which are currently used, hard metal grain or hard metal chips is/are mixed into the cast steel in order to increase the wear strength, a difficulty arises of economically recovering the two different metal materials.
There is therefore a desire to firstly solve the problem with the over-rapid wear-down, the currently far too short wear length, the random and uncontrollable exchange of wearing teeth which are not yet fully worn down, combined with the fact that certain wearing teeth are changed when the tooth holder has already suffered serious damage, and that the recovery in certain cases is both costly and complicated.
Patent specification SE 449,383 (U.S. Pat. No. 4,584,020) shows in FIG. 3 a digging or dredging tooth comprising a cast alloy and a wearing layer of a cast-in hard metal. Although this wearing tooth comprises an inner wearing layer, firstly this is arranged over the entire width of the tooth tip and is thus blunt, even as new, so that it does not have an optimal penetrating function, and secondly the wearing layer is disposed neither in the centre line of the tooth or in its two planes of symmetry A, B, so that the wear-down will make the wearing tooth still more blunt and ineffectual, i.e. it must either be discarded prematurely or it must be ground such that its wearing layer again ends up in the centre line.
The cast steel in the said SE 449,383 (U.S. Pat. No. 4,584,020) which is used has a carbon content of between 1.5% by weight and 2.5% by weight, which gives too soft a steel, so that the inner wearing layer, will be gradually exposed a further bit at a time, whereby the wearing layer will quite simply be broken off. This since the breaking strength is too low for the wearing layer to be able to withstand the loads without the support of the cast steel. Therefore, regardless of the fact that the wearing part has an inner wearing layer, the wear-down will be disadvantageously quick, since the wearing layer will actually be broken off in quite large fragments before it experiences any effectiveness-raising effect. In addition, it is maintained that a steel film with low carbon content (<0.20%) must be placed around the hard metal body. The melting point for the film must be 200-400° C. higher than the melting point for the cast alloy.
The nodular cast iron which is used in the prior art generally has a low hardness of around 38 HRC, and the wearing layer, which is a low-alloyed steel, has a hardness of between 40 and 53 HRC, which means that the low-alloyed steel matrix in the abovementioned wearing part only acquires approximately double the strength relative to a comparable cast iron product according to the prior art. Moreover, this is only a theoretical ratio, since the reality is that the wearing part, due to the brittleness of the wearing layer and the lack of supporting cast steel, which cast steel, as stated above, is too soft to be hard-wearing and is therefore worn away quickly, becomes still weaker. The way to solve this therefore remains an unresolved problem, which problem, despite long-lasting awareness thereof, has never satisfactorily been solved, in spite of the significant economic incentive as set out above. Based on the above prior art, it is clear that it has hitherto been felt that a hard metal should be cast into an iron alloy with relatively high carbon content in order to create a body, and in which prior art the said body is subsequently cast into an iron alloy with lower carbon content, for example according to U.S. Pat. No. 4,584,020.
Previous attempts at casting of low-alloyed steel have resulted in the dissolution of the hard metal in a bonding zone against the cast steel, and the formation in the said bonding zone of brittle tungsten-iron carbide fibres. Moreover, in this fusion of the cast steel and hard metal surfaces, any impurities or moisture can give rise to disadvantageous gas bubbles and hence cavities in the bonding zone inside the cast wearing part, which causes poorer adhesiveness and poorer strength in the said bonding zone and hence the above-stated uncontrollable splitting of the wearing surfaces into largish splinters or fragments, which very rapidly wears down the whole of the tooth neck, regardless of whether a hard metal is provided or not, to the point where the wearing tooth becomes ineffectual or the tooth holder is damaged.
The actual placement of the cast-in part, in this case the wearing layer of hard metal, in the casting mould in itself constitutes a problem, since the cast-in part moves away when the cast steel melt is poured down into the space for it in the casting mould. Previous solutions have involved, for example, various supports inside the said space, which supports were then melted and combined with the cast steel melt in the casting operation. It will be appreciated that this known method gives rise to a significant risk of the cast-in part moving from the desired position when the supports melt and, moreover, this melt of the supports forms an impurity in the cast compound, which alters the desired properties of the wearing part and the bonding zone between the cast-in part and the rest of the cast steel. For example, a poor adhesion can be caused, bubbles can appear and brittle metal mixtures can be formed in the cast steel in the said bonding zone during the casting of the wearing part.